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Treatise on Geomorphology Chapter · March 2013

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Provided for non-commercial research and educational use only. Not for reproduction, distribution or commercial use. This chapter was originally published in the Treatise on Geomorphology, the copy attached is provided by Elsevier for the author’s benefit and for the benefit of the author’s institution, for non-commercial research and educational use. This includes without limitation use in instruction at your institution, distribution to specific colleagues, and providing a copy to your institution’s administrator.

All other uses, reproduction and distribution, including without limitation commercial reprints, selling or licensing copies or access, or posting on open internet sites, your personal or institution’s website or repository, are prohibited. For exceptions, permission may be sought for such use through Elsevier’s permissions site at: http://www.elsevier.com/locate/permissionusematerial Fenton L.K., Ewing R.C., Bridges N.T., and Lorenz R. (2013) Extraterrestrial Aeolian Landscapes. In: John F. Shroder (ed.) Treatise on Geomorphology, Volume 11, pp. 287-312. San Diego: Academic Press. © 2013 Elsevier Inc. All rights reserved.

Author's personal copy 11.15 Extraterrestrial Aeolian Landscapes LK Fenton, Carl Sagan Center at the SETI Institute, Mountain View, CA, USA RC Ewing, University of Alabama, Tuscaloosa, AL, USA NT Bridges and R Lorenz, Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA r 2013 Elsevier Inc. All rights reserved.

11.15.1 11.15.1.1 11.15.1.1.1 11.15.1.1.2 11.15.1.2 11.15.1.2.1 11.15.1.2.2 11.15.1.3 11.15.1.3.1 11.15.1.3.2 11.15.2 11.15.2.1 11.15.2.1.1 11.15.2.2 11.15.2.2.1 11.15.2.2.2 11.15.2.2.3 11.15.2.2.4 11.15.2.3 11.15.2.3.1 11.15.2.3.2 11.15.2.3.3 11.15.2.4 11.15.2.4.1 11.15.2.4.2 11.15.2.4.3 11.15.2.5 11.15.2.5.1 11.15.2.5.2 11.15.3 11.15.3.1 11.15.3.2 11.15.3.3 11.15.4 11.15.4.1 11.15.4.2 11.15.4.3 11.15.5 11.15.5.1 11.15.5.2 11.15.5.3 11.15.6 References

Overview Mars Aeolian features Atmosphere and circulation Venus Aeolian features Atmosphere and circulation Titan Aeolian features Atmosphere and circulation Creation of Aeolian Depositional Landscapes Aeolian Sediment State Sediment state on Earth Mars Sediment state Sediment supply Sediment availability Transport capacity Venus Sediment supply Sediment availability Transport capacity Titan Sediment supply Sediment availability Transport capacity Depositional Sinks – Continuity at Basin Level Mars Venus and Titan Emergent Structures in Depositional Aeolian Landscapes Self-Organization Dune Field Patterns Bedform Scaling Erosional Landscapes Review of Deflation and Abrasion on Earth Deflation and Abrasion Elsewhere in the Solar System Comparison Between Earth and Other Bodies Unanswered Questions Mars Venus Titan Conclusions

288 289 291 291 294 294 296 296 296 297 297 297 297 298 298 298 298 299 299 299 299 299 299 299 300 300 300 300 301 301 301 301 303 303 304 304 305 306 306 306 307 307 307

Fenton, L.K., Ewing, R.C., Bridges, N.T., Lorenz, R., 2013. Extraterrestrial aeolian landscapes. In: Shroder, J. (Editor in Chief), Lancaster, N., Sherman, D.J., Baas, A.C.W. (Eds.), Treatise on Geomorphology. Academic Press, San Diego, CA, vol. 11, Aeolian Geomorphology, pp. 287–312.

Treatise on Geomorphology, Volume 11

http://dx.doi.org/10.1016/B978-0-12-374739-6.00308-0

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Glossary Aphelion The point in a body’s orbit around the Sun where it reaches its farthest distance from the Sun (contrast with perihelion). Baroclinic instability An instability occurring in rapidly rotating, stratified fluids with horizontal temperature gradients, such as the atmosphere and oceans. Baroclinic instability is responsible for shaping cyclonic and anticyclonic activity (i.e., storms) in the mid- and highlatitudes on Earth and Mars. Eccentricity A parameter describing the shape of a body’s orbit around another body. A value of 0 corresponds to a perfect circle; values greater than 0 and less than 1 correspond to increasingly oblate ellipses. Hadley circulation Global transport of air that drives much of the near-surface circulation on Venus, Earth, Mars, and Titan. On Earth, warm air rises near the equator and sinks in the subtropics; on the other worlds this pattern varies, in some cases dramatically. Hydrocarbons Organic compounds composed of hydrogen and carbon, most of which occur in crude oil deposits on Earth; Titan’s extensive sand seas are likely composed of hydrocarbon sands that precipitated out of the atmosphere. Intertropical Convergence Zone (ITCZ) The low-latitude region in the atmosphere where air rises, beginning the circulation that carries warm air poleward. On Earth and

Venus the ITCZ generally occurs near the equator; on Mars and Titan this point of convergence varies in latitude seasonally. Obliquity A parameter describing the tilt of a body’s rotational axis; presently Earth’s obliquity is 23.451. Perihelion The point in a body’s orbit around the Sun where it reaches its closest approach to the Sun (contrast with aphelion). Reticulate bedforms Bedforms B5–30 m in wavelength occurring in high altitude dusty regions on Mars, thought to be composed of dust aggregates. Thermal tide Diurnal and semidiurnal atmospheric tides driven by solar heating, often migrating with the sun’s path across a planetary body. Although observed on Earth, thermal tides on Mars are much stronger as a result of that planet’s thin atmosphere, such that net low pressure and convergence of air (and therefore wind) occurs at the subsolar point on the surface. Transverse aeolian ridge (TAR) Relatively bright bedforms common on Mars, with a morphology similar to ripples but with wavelengths similar to those of transverse dunes. Valles Marineris A vast system of canyons on Mars extending more than 4000 km nearly parallel to the planet’s equator. In places the canyon is more than 200 km wide and up to 7 km deep.

Abstract Over the last half century, spacecraft visits to the many worlds in our Solar System have revealed that the surfaces of no fewer than four planetary bodies are subject to aeolian processes. These worlds beyond Earth include the planets Venus and Mars, as well as a moon of Saturn, Titan. Each body shows the influence of the wind in a unique way, with our understanding strongly controlled by the quantity and type of data returned from spacecraft investigations. Of these worlds, the best studied is Mars, which is prone to dust storms and is freckled with bedforms, yardangs, and wind streaks. Venus is spanned by thousands of wind streaks, but bedforms and yardangs are rare (or unresolved by available data). Now known to be the sandiest world in the Solar System, Titan’s equator is belted by vast sand seas made of hydrocarbon grains. Although conditions vary widely across the Solar System, the depositional and erosional processes acting on these worlds are in many ways similar to those observed on Earth. As a result, concepts and methods developed for studying terrestrial aeolian landforms can be applied to their planetary counterparts. The factors controlling dune field sediment state are the same (sediment supply, availability, and wind transport capacity), although they are commonly controlled by very different processes on other worlds. Emergent structures (i.e., bedforms) are self-organized; thus, they are controlled by dune-scale processes regardless of where they form, so that extraterrestrial dune field patterns may be analyzed with the same techniques used on Earth (e.g., pattern analysis, gross bedform-normal transport). Scaling relationships for elementary bedforms have been developed, which correlate well with bedforms formed under varying conditions. However, these relations do not appear to hold everywhere (particularly on Venus and Titan); this may be a result of low resolution data that cannot resolve elementary features of the predicted size. Although not well observed on Venus or Titan, deflation and abrasion are active processes on Mars, scouring dusty surfaces and eroding materials into yardangs and ventifacts. However, it is likely that the timescales of erosion are much longer on Mars than on Earth. Continuing studies show that aeolian landscapes beyond Earth can appear at once hauntingly familiar and exotically alien. Although there are many fundamental unanswered questions about aeolian processes on Mars, Venus, and Titan, it is clear that this juxtaposition occurs because the processes that produce aeolian landscapes on Earth also operate elsewhere, but they do so under vastly different conditions and over very different time- and length-scales.

11.15.1

Overview

All worlds in the Solar System with substantial atmospheres and observable surfaces show the effects of the wind

interacting with the ground. Since the 1960s, spacecraft instruments have revealed the presence of aeolian features, first on Mars, then on Venus, and most recently and dramatically, on Titan, the largest moon of Saturn (see Figure 1). Despite

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widely varying geological histories, the work of the wind has strongly influenced, and even dominated, these extraterrestrial landscapes. Venus, Earth, Mars, and Titan all have widely varying planetary properties related to wind-blown sediment transport, such as gravity, atmospheric composition, and air pressure (see Table 1). Laboratory experiments predicted that, like on Earth, sand-sized grains (B100–300 mm) are the most likely particles to saltate (e.g., Greeley et al., 1980; Greeley and Iversen, 1985 and references within), indicating that, despite their differences, aeolian processes have the potential to play a significant role in the sedimentary history of these worlds. Since these early studies, spacecraft data have borne out these predictions. On Mars, the many dust storms, dunes, ripples, yardangs, dust mantles, wind streaks, and exhumed surfaces suggest that aeolian deposition and erosion may be the dominant processes sculpting the landscape today (see Section 11.15.1.1). Although only two dune fields have been identified on Venus, thousands of wind streaks attest to the work of the wind on the planet’s surface (see Section 11.15.1.2). Finally, and rather strikingly, Titan is host to the largest accumulation of sand seas in the Solar System, which collectively span B2 times the estimated sand sea coverage on Earth (see Table 1 and Section 11.15.1.3).

Our knowledge of aeolian features beyond Earth comes from a long history of spacecraft missions in the Solar System (see Table 2). Early flyby missions failed to provide enough evidence for aeolian activity; rather, it was the first spacecraft mission devoted to mapping the surface of each of these planetary bodies that revealed the presence of bedforms and other aeolian features. In the absence of field study, remote sensing data drive scientific inquiry about these worlds. As a result, the current knowledge and understanding of such aeolian landscapes is directly related to the amount and type of data returned from spacecraft. Cameras acquiring digital images in the visible and infrared, as well as synthetic aperture radar (SAR), provide imagery that can be used to interpret surface morphology, but only to the resolution limit of these instruments. Infrared spectrometry can be used to determine surface mineralogy, and thermal infrared radiometry can be used to infer thermal properties of surface particles (including grain sizes of loose particulates). Neutron spectrometry can determine the location of near-surface ground ice, helping to determine local climate regimes. Radar can locate underground bedding in fine-grained and icy materials, providing a crude look at the stratigraphy of some formations. In contrast to what is observed from space, landed missions provide a detailed look at a very limited region, and bear instruments that generally provide more thorough measurements of surface and atmospheric characteristics. Not all types of instruments have visited all of the worlds in the Solar System, making intercomparison challenging. As a result, the study of extraterrestrial aeolian processes is still highly dependent on the vast knowledge acquired from terrestrial field and laboratory studies.

11.15.1.1 Figure 1 The four aeolian worlds in our Solar System: Venus, Earth, Mars, and Titan. Courtesy of NASA/JPL-Caltech.

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Mars

For centuries, telescopic studies of the ‘Red Planet’ have caught the imagination of astronomers, with the first attempted maps

Table 1 Basic characteristics of the four aeolian worlds in the Solar Systema Planetary body

Venus

Earth

Mars

Titan

Mean distance from Sun (AU) Diameter (km) Gravity (m s  2) Mean surface atmospheric pressure (bar) Mean surface air temperature (1C) Atmospheric composition (no trace gasses)

0.72 12 104 8.87 91 464 96.5% CO2, 3.5% N2

1 12 742 9.81 1.01 15 77% N2, 21% O2

9.58 5150 1.35 1.47  178 95% N2, 5% CH4

Total topographic relief (km) Estimated minimum threshold friction speed (m s  1) Estimated dune field coverage (106 km2) Estimated dune field coverage (% surface area, including oceans)

13.7 0.02b

19.8 0.2b

1.52 6779 3.71 0.007  63 95% CO2, 2.7% N2, 1.6% Ar 29.4 2.0b

0.0183c 0.004c

5d 0.98d

0.9e 0.62e

10f 12.5f

a

Unless otherwise noted, values come from NASA’s website. Greeley and Iversen (1985). c Greeley et al. (1995). d Livingstone and Warren (1996). e Fenton and Hayward (2010). f Le Gall et al. (2011). Source: http://nssdc.gsfc.nasa.gov/planetary/planetfact.html b

B0.5 0.04b

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Table 2 Spacecraft missions that have significantly advanced our understanding of planetary aeolian landscapes Mission

Date

Instruments important to aeolian research

Minimum pixel sizea

Venus Magellan Venus Express

1990–1994 2006–present

SAR Venus monitoring camera (VMC) Visible and infrared thermal imaging spectrometer (VIRTIS)

B75 4200

Mars (Orbiters) Mariner 9

1971–1972

Viking Orbiter 1 Viking Orbiter 2

1976–1980 1976–1978

Visual imaging system (VIS) Infrared interferometer spectrometer (IRIS) Visual imaging subsystem (VIS) Infrared radiometer for thermal mapping (IRTM)

4100 4B107 000 48 48000

Mars Global Surveyor

1996–2006

Mars Odyssey

2001–present

Mars Express

2003–present

Mars Orbiter camera (MOC) Mars Orbiter laser altimeter (MOLA) Thermal emission spectrometer (TES) Thermal emission imaging system (THEMIS) Mars Odyssey neutron spectrometer (MONS) High resolution stereo camera (HRSC) Visible and infrared mineralogical mapping spectrometer (OMEGA) High resolution imaging science experiment (HiRISE) Context imager (CTX) Compact reconnaissance imaging spectrometer (CRISM)

41.5 B168 3000  B6000 VIS: 18, IR: 100 B550 000 410 4300 40.30 B6 418

Mars Reconnaissance Orbiter 2006–present

Mars (Landers) Viking Lander 1 Viking Lander 2

1976–1982 1976–1980

Viking meteorology instrument system (VMIS) Stereo cameras

Mars Pathfinder

1997

Imager for Mars Pathfinder (IMP)

Mars Exploration Rovers Opportunity Spirit

2004–present 2004–2010

Phoenix Mars Lander

2008

Panoramic camera (Pancam) Miniature thermal emission spectrometer (Mini-TES) Microscopic Imager (MI) Surface stereo imager (SSI) Optical microscope Meteorological station (MET)

Mars Science Laboratory Curiosity

Titan Cassini-Huygens

2012–present

2004–present

ChemCam Chemistry and Mineralogy (CheMin) Mars Hand Lens Imager (MAHLI) Mast Camera (MastCam) Remote Environmental Minitoring Station (REMS) Huygens probe Radar Visible and infrared mapping spectrometer (VIMS) Imaging science subsystem (ISS)

30 mm

4 mm

o700 mm 413.9 mm 4150 mm

4350 4250 4B300

a

Units are in m/pixel unless otherwise listed; lander instruments have variable spatial resolutions and are not listed.

dating back nearly four centuries. Early observers noted temporally varying features on Mars, including shrinking and growing polar caps, as well as ‘‘white’’ and ‘‘yellow’’ clouds. It was not until the twentieth century that each of these was identified as water clouds and dust storms, respectively (e.g., see the review in Martin et al., 1992), and aeolian processes were considered quantitatively (Ryan, 1964). In 1971, Mariner 9, the first spacecraft ever to orbit another planet, arrived in the midst of a planet-encircling dust storm that obscured nearly all surface features. Further inspection of these images, once the dust settled, led to discovery of a few dune fields and many wind streaks (McCauley, 1973; Sagan et al., 1972). Further imagery from Mariner 9 and the two Viking orbiters (operating from 1976 through 1981) revealed

the prevalence of erosional and depositional aeolian environments on Mars, including dune fields, dust-mantled terrain, and wind-eroded surfaces (McCauley, 1973; Ward et al., 1985). The sudden influx of data prompted many laboratory and field studies, as well as investigations of terrestrial remote sensing of aeolian environments, in order to better understand the conditions that produced the martian landscapes (e.g., Greeley and Iversen, 1985 and references within). These early missions and studies led to a general understanding of Mars’ geological past and how it differs from that of Earth. With regard to aeolian processes, for example, the lack of present-day oceans means there is no global sink for sediment, and the lack of plate tectonics means that sediment recycling has happened only at the surface through

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impact, fluvial, aeolian, and glacial processes. As a result, Mars’ surface preserves a long record of aeolian deposition and erosion that undoubtedly contains billions of years of sedimentary history. Starting in the 1990s, a succession of missions provided an abundance of high resolution imagery that revolutionized our understanding of martian geology. Beginning with the Mars Orbiter camera (MOC) on the Mars Global Surveyor, high resolution images revealed a world covered in layered sediments that have undergone periods of extensive erosion and wind-eroded terrains rife with yardangs and bedforms (e.g., Malin and Edgett, 2001). Subsequent orbital missions have continued to refine our understanding of martian landscapes with better image coverage, surface mineral determination from spectroscopy, and higher-resolution images (see Table 2). Landed spacecraft have demonstrated that aeolian grains (both dust and saltatable sand) are common in many regions (e.g., Sullivan et al., 2008), and that aeolian processes are responsible for sculpting the surfaces and producing some of the sedimentary strata observed in situ (e.g., Grotzinger et al., 2005). It is likely that the work of the wind changes the shape of the landscape more than any other currently active process, and that it has been active throughout the planet’s history.

11.15.1.1.1

Aeolian features

From orbit, the effect of the wind on the surface is clear. Recent surveys show that there are roughly 2000 low albedo dune fields on Mars, collectively covering at least B904 000 km2 (Hayward et al., 2007; Fenton and Hayward, 2010). This area may seem vast, but it is far smaller than the total estimated dune field coverage on Earth (see Table 1). Figure 2(a) shows that the bulk of the dunes (spanning B750 000 km2; Hayward et al., 2008) are in northern polar sand seas that ring the polar cap (Figure 3(a) shows a small portion of one of these sand seas). Most of the rest of the dunes are located in low-lying areas, typically on crater floors in the southern highlands (see Figure 3(b)) and in Valles Marineris, a broad canyon system lying along Mars’ equator. In addition to the large dark dunes, many areas are covered by smaller, brighter bedforms, generically termed ‘transverse aeolian ridges’ (TARs, see Figure 3(c)), because it is unclear in many cases whether they are unusually large ripples or small dunes (e.g., Balme et al., 2008; for further discussion on bedform types, see Section 11.15.3.3). Vast yardang fields indicate that aeolian erosion has stripped a large volume of sediment in some areas (see Figure 3(d) and discussion in Section 11.15.4.2). The yardangs commonly occur in complex patterns and have such unusual morphologies (e.g., Bradley et al., 2002), that terrestrial analogs are just now being identified (de Silva et al., 2010). Even in locations where no streamlined erosional remnants are left behind, wind erosion is thought to be responsible for modifying the surface, perhaps quite dramatically (e.g., Armstrong and Leovy, 2005; Grant et al., 2008). Wind streaks in the lee of topographic obstacles may be either depositional or erosional (Thomas et al., 1981); some map out sand transport pathways (e.g., Edgett, 2002). Imagery from HiRISE, with a resolution down to 30 cm/pixel, has revealed aeolian features undetectable in previous data sets, including ripples on dunes and, surprisingly, bedforms at extremely high elevations (B10–21 km above the datum), where the atmospheric pressure is less than 1 mbar (Bridges et al.,

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2007). Dust aggregates, rather than clastic sand grains, may be responsible for creating the small (B5–30 m wide) poorly understood ‘‘reticulate’’ bedforms occurring in these unusual locations (Bridges et al., 2010). Further data from upcoming missions will likely generate yet more discoveries on the wide variety of aeolian features that pervade the surface of Mars. Mars is prone to dust storms that can range in size from tens to thousands of kilometers; the largest can inundate nearly the entire planet (see Figure 4), last for months, and kick fine dust several tens of kilometers into the atmosphere. These planet-encircling dust storms occur in the southern spring and summer, with the most dramatic examples in the past few decades occurring in 1971, 1977, 2001, and 2007. The cause of these immense storms is not well understood, although detailed study of the more recent events has revealed that they are comprised of a series of local and regional dust storms (those spanning either less or more than 1.6 106 km2, respectively) that trigger one another and contribute to a highlevel atmospheric haze that constitutes the ‘global’ aspect of the storm (Cantor, 2006). Smaller storms are common at the polar cap edges, near areas of high relief, and in the midlatitudes, although the depletion of local dust reservoirs appears to affect the pattern of storm formation over time (Cantor et al., 2001). The brightest regions on Mars (apart from the polar caps) are created from mantles of dust, perhaps more than 20 m thick (Mangold et al., 2009), which drape over topography; one such region (Arabia Terra) is prominent in Figure 4. These areas correspond to regions of relatively low wind strength, which effectively trap dust that falls out as dust storms weaken (Greeley et al., 1993), and they may be regarded as martian analogs for loess (albeit they are less consolidated and composed of finer grains than terrestrial loess). Beyond the edges of these deposits, the redistribution of dust by the large storms can cause dramatic shifts in regional albedo patterns over time, as thin deposits of relatively bright dust accumulate on the relatively dark surface and are then swept away by strong wind gusts and dust devils (e.g., Geissler, 2005). This process is notable on a smaller scale as well, causing problematic dust buildup on rover solar panels and erasing rover tracks (Geissler et al., 2010).

11.15.1.1.2

Atmosphere and circulation

Of all planetary bodies with aeolian features, Mars has the thinnest atmosphere (see Table 1). However, in some ways it is more like that of Earth than the other aeolian worlds. Global wind circulation patterns are partially dictated by Hadley cells and baroclinic instability, produced by an obliquity and rotation rate (25.191 and 24 h, 39.6 m, respectively) similar to that of Earth (23.451 and 24 h, respectively). The differences between the two planets, however, are quite profound. Mars is prone to dust storms that infrequently overwhelm large regions, and every few years even grow to encompass the whole globe. The CO2 atmosphere partially condenses at high latitudes during the winter, leading to a repeatable 30% change in global mean air pressure over the course of a year; in contrast, Earth’s most abundant condensable atmospheric constituent, water vapor, varies spatially and temporally by a few percent. Such a dramatic seasonal shift in air pressure can influence particle entrainment by inhibiting saltation during periods of low air pressure. As

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90°

45°

0°

–45°

–90° 0° E

(a)

90° E

180° E

270° E

90° Al Uzza Undae 45°

0°

–45°

Menat Undae

–90° 0° E

(b)

90° E

180° E

270° E

180° E

270° E

90°

45°

0°

–45° - Dunes identified in SAR data - Areas with high known dune correlation - Areas with moderate known dune correlation

–90° 0° E

90° E

(c)

Figure 2 Locations of dune fields on (a) Mars. Reproduced from Hayward, R.K., Mullins, K.F., Fenton, L.K., et al., 2007. Mars Global Digital Dune Database and initial science results. Journal of Geophysical Research 112, E11007. doi:10.1029/2007JE002943; Hayward, R.K., Fenton, L.K., Tanaka, K.L., et al., 2008. Mars Global Digital Dune Database: distribution in North Polar region and comparison to Equatorial region. Proceedings of Lunar and Planetary Science Conference XXXIX, League City, TX, Abstract #1208, and Fenton, L.K., Hayward, R.K., 2010. Southern high latitude dune fields on Mars: morphology, Aeolian inactivity, and climate change. Geomorphology 121, 98–121, (b) Venus. Adapted from Greeley, R., Bender, K.C., Saunders, R.S., Schubert, G., Weitz, C.M., 1997. Aeolian processes and features on Venus. In: Bougher, S.W., Hunten, D.M., Phillips, R.J. (Eds.), Venus II: Geology, Geophysics, Atmosphere, and Solar Wind Environment. University of Arizona Press, Tucson, pp. 547–589. Topographic map by Steve Albers, Calvin Hamilton, and A. Tayfun Oner (http://laps.noaa.gov/albers/sos/sos.html), and (c) Titan. From Stephan, K., Jaumann, R., Karkoschka, E., et al., 2010. Mapping products of Titan s surface. In: Brown, R.H., Lebreton, J.-P., Waite, J.H. (Eds.), Titan from Cassini-Huygens. Springer, New York, pp. 489–510. In the case of Titan, dunes have been identified in the regions marked in white; blue and brown regions correspond to terrains that are highly and moderately correlated with dune fields, respectively, and are therefore likely to contain dunes. Courtesy of NASA/JPL/Space Science Institute.

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1 km

(a)

(b)

293

1 km

1 km

(c)

(d)

1 km

Figure 3 Examples of aeolian features on Mars. (a) Networked dunes in the North Polar Sand Seas (image: CTX P22_09752_2602, 211.01 E, 80.31 N); (b) a typical intracrater dune field in the southern highlands (image: THEMIS VIS V26522016, 4.71 E, 56.21 S); (c) TARs in an ancient river valley (image: MOC E02/02651, 316.91 E, 27.41 S); (d) the edge of a yardang field (image: THEMIS VIS V26179002, 187.71 E, 8.71 E).

Mars • Global dust storm

June 26, 2001

Hubble space telescope • WFPC2

September 4, 2001

NASA, J. Bell (Cornell), M. Wolff (SSl), and the Hubble heritage team (STScl/AURA) STScl-PRC01-31 Figure 4 Hubble Space Telescope images of Mars just prior to the onset of the 2001 global dust storm (just after the southern spring equinox) and in the midst of the storm 2 months later (during southern spring).

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on Earth, high relief can lead to strong topographically driven local winds; but on Mars such flows are more likely to dominate over dynamically driven wind patterns (e.g., weather systems). Because saltation thresholds are higher on Mars than on Earth, aeolian activity may be controlled to some extent by proximity to high relief terrain, which may be required in some locations to accelerate winds beyond the entrainment threshold. Figure 5(a) shows a highly idealized concept of zonalmean circulation throughout a year on Mars. On Earth, two Hadley cells form near the equator, as latitudinal gradients in solar heating force air to rise at the Intertropical Convergence Zone (ITCZ) and sink in the subtropics. On Mars, the quick thermal response of the surface (enhanced by the lack of oceans) leads to an ITCZ that shifts significantly in latitude with the seasons. This leads to Hadley cells of unequal sizes; at the solstices air rises in the summer hemisphere and sinks in the winter hemisphere, leading to a cross-equatorial Hadley cell that dominates meridional circulation (shown in green in Figure 5(a)). Return flow near the surface dictates the mean wind pattern (large white arrows in the Hadley cells), which commonly affects aeolian features on the ground. Planetary rotation creates a Coriolis effect that deflects the surface winds toward the west as they approach the equator, and differences in surface topography divert the air differently, depending on whether the return flow is arriving from the north or south. This pattern of return Hadley flow is best reflected in the orientations of kilometers-long wind streaks, which are thought to be created by deposition of relatively bright dust in the lee of topographic obstacles, and are prevalent throughout the low- and mid-latitudes of Mars (Magalha˜es, 1987; Greeley et al., 1993; Fenton and Richardson, 2001). Mars has a substantial orbital eccentricity (0.0935 vs. Earth’s 0.0167), with perihelion occurring during southern summer. In addition, surface elevations in the southern hemisphere are a few kilometers higher than those in the north. Both effects lead to more vigorous Hadley circulation during southern summer. Because of this enhanced circulation, the largest dust storms tend to occur during southern summer. Orbital parameters, such as eccentricity, obliquity, and season of perihelion, vary over time much more dramatically than on Earth (Laskar et al., 2004), and strongly force the global climate. Atmospheric modeling on a global scale suggests that although wind stresses may be strongly influenced by varying orbital parameters, influencing the seasonal dust storm pattern (Haberle et al., 2006), wind patterns remain remarkably stable (Fenton and Richardson, 2001; Haberle et al., 2003). The Hadley cells alone do not account for all winds experienced at the surface of Mars. Seasonal deposition of CO2 ice at each pole in the winter reaches down to B551 N in the northern hemisphere and B501 S in the southern hemisphere. Strong thermal contrasts at the edges of these polar caps create wintertime jet streams that are responsible for westerly winds and fronts, particularly in the smooth northern plains where topography does not impede their passage. Springtime sublimation of these seasonal ice caps leads to off-cap ‘sublimation winds’ that may locally entrain particles. A continuous thermal tide created by solar heating causes a local low pressure zone wherever the Sun is overhead, causing a regular diurnal shift in wind direction in most locations. Local

topography, created by crater rims, volcano slopes, and canyon walls produce intense slope winds, leading to upslope flow during the day and downslope flow at night; it is these winds that, until recently, have prevented landed missions from descending into areas with views of Mars’ spectacular topography. Finally, strong daytime heating of the surface leads to convective activity that produces wind gusts and dust devils that are superimposed on the mean flow created by the processes listed above. As on Earth, circulation patterns on Mars are determined by many different air flows that vary significantly with location and season.

11.15.1.2

Venus

Venus probes in the 1970s revealed the atmospheric pressure at the surface to be some 90 bar (equivalent to the pressure at a depth of nearly 900 m in water on Earth). This value translates, given the molecular weight of the predominantly CO2 atmosphere and the high temperatures, to an air density of about 64 kg m3 (about 50 times that of sea-level air on Earth). This high density might lead one to expect aeolian transport to be rather easy, so that aeolian features would be widespread on the surface. However, in contrast to the abundant sand drifts seen in images from the surface of Mars by the Viking Landers in 1976, images returned from the torrid Venusian surface by several Soviet Venera probes showed relatively little fine-grained material (although there were some indications that the landings kicked up clouds of dust). Because the surface is hidden from view (at optical wavelengths) by a thick cloud layer extending from 50 to 70 km in the atmosphere, there was no early orbital camera survey comparable to the Mariner 9 and Viking orbiter surveys of Mars. Only two areas of resolvable dunes and one possible yardang field were identified in Magellan radar imagery (e.g., Greeley et al., 1992, 1997; Weitz et al., 1994). However, these surveys found that wind streaks are plentiful, and include both those forming in the lee of pre-existing obstacles (similar to those found on Mars), as well as several that were created by the downwind dispersal of ejecta during crater impacts. Identification of dune fields is dependent on spatial resolution and radar look angle, limiting discovery to larger bedforms and fortuitous spacecraft geometry. It is likely that future missions with improved instruments will greatly enhance the known number of both depositional and erosional aeolian features on Venus.

11.15.1.2.1

Aeolian features

Figure 2(b) shows the two dune fields identified on Venus, which are formally named Menat Undae and Al-Uzza Undae (‘unda’ is the Latin term used for a dune on a planetary surface), although the original names Aglaonice and Fortuna-Meshkenet, respectively, are more commonly used in the literature. At 251 S, 3401 E, Menat Undae cover some 1300 km2 at the edge of an outflow deposit from Aglaonice crater. It is possible that there are many more dune fields that could not be directly identified in Magellan data. For example, in many areas where no resolvable dunes were detected, it was noted that the radar echo had a substantially different strength when the area was observed from one side versus another. One explanation of this radar asymmetry is that unresolved

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‘‘microdunes’’ may be present (Greeley et al., 1984a; Weitz et al., 1994), wherein the asymmetry is due to shallow stoss slopes being prominent from one direction against the steeper slip faces seen from the other direction.

The best resolved dunes are Al-Uzza Undae, which are located at a high northern latitude (671 N, 911 E), span 17 000 km2, and lie in a valley between Ishtar Terra and Meshkenet Tessera (see Figure 6). They appear to be transverse

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that not all wind streaks follow the equatorward Hadley circulation; some may be influenced by local topography or other atmospheric flows that are poorly understood, but may be relevant to sediment transport (Dobrovolskis, 1993). The two dune fields and yardangs appear to reflect local, rather than global winds. For example, Al-Uzza Undae are mainly transverse to a southeasterly wind, despite being in the northern hemisphere, where the mean wind streak trend indicates northerly winds. Unfortunately, a sample of three is too poor statistically to be informative about global or regional circulation.

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11.15.1.3

Figure 6 A portion of Al-Uzza Undae on Venus from the Magellan SAR. The orientations of radar-bright wind streaks suggest the dunes are transverse. Image courtesy of NASA Planetary Data System Imaging Node, U.S. Geological Survey, Flagstaff, AZ.

bedforms, in that there are several bright wind streaks visible in the region, which seem generally orthogonal to the dunes. Overlapping radar imagery 8 months apart found that there were no morphologic changes, so the 200–500 m wide dunes did not migrate (unsurprisingly, given that terrestrial dunes of this size move quite slowly) or become otherwise modified during that time period (Weitz et al., 1994).

11.15.1.2.2

Atmosphere and circulation

Venus orbits the Sun in 224.7 terrestrial days, but the time from sunrise to sunrise lasts for 116.8 terrestrial days; thus a Venusian year is 1.92 Venusian days long. The planet’s orbital eccentricity is low at 0.007, and its axial tilt is 177.41 (one may also regard this as an obliquity of 2.61 with a retrograde spin), so seasons are effectively nonexistent. The thickness of the atmosphere (see Table 1) creates an extreme greenhouse effect that traps heat, keeping surface temperatures from varying significantly with latitude or time of day. Measurements of wind streak orientations provide some clues regarding atmospheric circulation on Venus (Greeley et al., 1997). At the surface, air generally flows toward the equator in both hemispheres at all latitudes, with the southern hemisphere wind having a westerly component that may be deflected by regional topography. Overall the wind streaks appear consistent with hemispheric Hadley cells, such that air rises at the equator and descends at the poles (see Figure 5(b)). Atmospheric observations, however, suggest that Hadley circulation at altitude reaches only B7601 latitude (e.g., Svedhem et al., 2007); it seems unclear why equatorward flow at the surface extends to the poles. These near-surface circulations also contrast sharply with observed high altitude super-rotating clouds, which circle the planet approximately every 4 Earth days. It should be noted

Titan

Titan is the second-largest satellite in the Solar System and the only one bearing a substantial atmosphere. In most respects, Titan’s environment is very different from that of the terrestrial planets (see Table 1). Low temperatures at this distance from the Sun produce conditions such that most of the surface materials (thought to be mainly water ice) are mechanically similar to rocks on the terrestrial worlds; fluids on the surface are composed of hydrocarbons, likely including methane and ethane (e.g., Brown et al., 2008). As with Venus, Titan’s surface is obscured from view in visible light, and has been observed from space only through a radar and a few narrow windows in the near infrared, most notably from the spacecraft Cassini (see Table 2). Decades ago Greeley and Iversen (1985) proposed that Titan’s atmosphere was substantial enough that dunes might form on the surface; however, it was not until the Cassini mission imaged the surface via radar that the aeolian dunes became apparent (Lorenz et al., 2006). Because of the limited spatial resolution of Cassini’s data and the infrequent data capture (Cassini orbits Saturn and only occasionally flies close enough to Titan to obtain high quality data), the study of aeolian landscapes there is still in its infancy.

11.15.1.3.1

Aeolian features

Cassini’s initial optical reconnaissance of Titan showed ‘streaky’ boundaries between light and dark terrain but at first it could not be determined unambiguously whether these features were the result of aeolian or fluvial transport (Porco et al., 2005). Hundreds of dark streaks were resolved in early radar imaging of dark terrains, but it was a near-equatorial flyby of the darkest areas in 2005 that made it clear that these were aeolian dunes (Lorenz et al., 2006). During this flyby, Cassini observed terrain that was in many places completely covered by what is now known to be sand-sized sediment. The equatorial groundtrack of the spacecraft showed that the dunes were oriented east–west, and illumination broadside-on provided many opportunities to detect topographic glints (see Figure 7). These glints clearly showed the dunes as positive relief features that were much more extensive and higher than flybys prior to 2005 indicated. The newer data also covered the Huygens probe landing site (10.21 S, 192.41 W), and in fact two dunes seen in the distance during descent were instrumental in correlating the Huygens optical imaging with the orbital radar (e.g., Lunine et al., 2008). Continuing data acquisition has led to the estimate that Titan’s dunes, as shown in Figure 2(c), are confined to within 301 of the equator and cover B12.5% of Titan’s

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Figure 7 Titan dunes from the Cassini SAR, centered at B121 S, 1001 W. Note the subtle deflection around bright features in the lower right and the low defect density. Courtesy of NASA/JPL-Caltech.

surface (Le Gall et al., 2011). Titan’s dunes appear to be similar in length, height, and spacing to the giant linear dunes in the Namib sand sea (Lorenz et al., 2006; Radebaugh et al., 2008, 2010). Some network dunes are noted, suggesting that complex wind regimes occur in specific locations, but predominantly the dunes are linear (see Figure 7). Comparing Titan dunes with terrestrial analogs has been a popular and powerful approach (e.g., Radebaugh et al., 2010) and has prompted re-examination of terrestrial features (e.g., Rubin and Hesp, 2009).

11.15.1.3.2

Atmosphere and circulation

Titan orbits Saturn effectively synchronously with a period (i.e., length of day) of 15.945 Earth days, with a spin axis tilted only 0.31 from its orbital plane around Saturn. More important to global circulation is Saturn’s orbit around the Sun, which has a period of 29.46 terrestrial years and an axial tilt of 26.731. Thus a ‘year’ on Titan is nearly 30 Earth years long, and seasons are produced by Saturn’s obliquity rather than its own. Furthermore, Saturn’s orbit has an eccentricity of 0.056, such that southern hemisphere summers on Titan, like those on Mars, are shorter but more intense. Our understanding of general circulation on Titan is based largely on atmospheric modeling. As with Venus, Titan’s atmosphere super-rotates at altitude, and this flow does not appear to influence the surface. Circulation near the surface on Titan seems to be fairly simple: solar heating produces Hadley cells that cause air to rise on either side of the ITCZ (Flasar et al., 2010 and references within). Titan does not rotate fast enough to produce mid- and high-latitude baroclinic instability, so the Hadley cells dominate meridional circulation and extend to high latitudes. At each solstice a single cell is thought to exist, rising in high latitudes in the summer hemisphere and sinking near the pole in the winter hemisphere (see Figure 5(c)). Surface winds are created by return flow of each cell, producing prevailing winds with northerly components during the southern summer and southerly components during the northern summer. The net convergence of these winds may be responsible for the accumulation of dunes at the equator. As on Mars and Earth, winds are deflected slightly by the Coriolis force so that there is an easterly component to the flow as air flows approach the equator, and a westerly component as the air flows approach the mid-latitudes. It is yet unclear what role topography and surface roughness

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play on the global wind patterns on Titan (e.g., Tokano, 2008), although regional winds, as reflected in dune orientation, do appear to be influenced by topography (Lorenz and Radebaugh, 2009). Based on their interactions with local topography (see Figure 7), the equatorial dunes appear to be extending from west to east. As shown in Figure 5(c), this direction of transport is in opposition to predictions of the average winds from general circulation models that produce resultant drift directions toward the west at the low latitudes where dunes occur (e.g., Tokano, 2008). Recently, Tokano (2010) produced a reasonable solution to the disparity between modeling efforts and observations: although low-latitude surface winds appear to blow with an easterly component for most of the year, a brief interval of strong westerlies (1–1.5 m s1) occurs around the equinoxes. If the saltation threshold is higher than expected (4B30 times the estimate in Table 1), then it will respond only to these stronger winds and the dune pattern will reflect not the average winds but rather these intense, short-lived westerlies.

11.15.2

Creation of Aeolian Depositional Landscapes

11.15.2.1

Aeolian Sediment State

The creation of an aeolian depositional landscape requires a supply of sediment that is available for transport by a wind of sufficient strength. Sediment supply, sediment availability, and wind transport capacity are three independent components of an aeolian system that together define the sediment state of a depositional landscape (Kocurek and Lancaster, 1999). As a result, the sediment state as a function of time reflects the climatic, eustatic, and tectonic conditions that give rise to a landscape at the field-, basin-, or planetary-scale, depending on the spatial and temporal range of interest (Beveridge et al., 2006; Kocurek et al., 2007).

11.15.2.1.1

Sediment state on Earth

On Earth, water-related processes generate much of the aeolian sediment supply either through erosion and weathering with the creation of siliciclastic sediment, or through the precipitation of chemical sediments such as gypsum and carbonate. Typically, fluvial, alluvial, glacial, coastal, and lacustrine systems generate the siliciclastic aeolian sediment supply, with little input from primary aeolian deflation/erosion of weakly lithified sediment and bedrock. Sediment input to a dune field may be contemporaneous, lagged in time, or a mixture of both (Kocurek and Lancaster, 1999). Contemporaneous influx is sediment that is generated and immediately available for transport into a dune field, and lagged influx is sediment that is stored (e.g., buried), which later becomes available for transport. For example, sediment created by fluvial erosion and transported to a coastline may be blown directly into a coastal dune field (contemporaneous influx) as well as stored under water on the continental shelf. During a sea-level lowstand, the stored submarine sand becomes reworked into aeolian sediment (lagged influx) while influx to the dune field from the river via coastal processes continues (contemporaneous influx). Cycles of sediment supply, sediment storage, and influx may be

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linked to Croll–Milankovich-driven climatic–eustatic cyclicity. On longer timescales, tectonic uplift may be a primary influence on the generation of aeolian sediment. On Earth, the dominant factor controlling sediment availability is water: the presence of precipitation, groundwater, and vegetation all serve to limit exposure of sand grains to the wind (Swezey, 2003). In addition, precipitated or biotic crusts and coarse-grained lags (surface armoring) also limit the availability of sediment for a given wind. In the example above, fluvially derived sediment stored on the continental shelf during a sea-level high stand is availability-limited; however, in another example, primary dune sediment may be availability-limited during a humid period in which vegetation occupies a relict dune field (e.g., mid-Holocene Sahara Desert). Generally, wind transport capacity varies as a cubic function of the friction speed in excess of the saltation threshold (Bagnold, 1941). Given a sediment supply that is fully available for transport, for a given friction speed, a wind will become saturated quickly. Stronger and/or higher frequency transport events result in the rapid depletion of a sediment supply and the equally rapid development of a dune field (on the order of one year to tens of years), such that in many cases aeolian systems are supply- and availability-limited, rather than transport-limited (Kocurek and Lancaster, 1999).

11.15.2.2 11.15.2.2.1

Significant amounts of hydrated calcium sulfates, including gypsum, occur in the north polar sand seas, suggesting that water has played a role in the sedimentary history of the presently cold and dry high northern latitudes (Langevin et al., 2005). However, it is unclear how dune sands were originally formed, because no single mechanism stands out as one that creates a majority of the aeolian sediment supply. Although water may have played a prominent role in the creation of sediment prior to the Amazonian epoch (B3 Ga – present; Pollack et al., 1987; Andrews-Hanna et al., 2007), the apparent absence of abundant liquid water for the past few billion years suggests that other mechanisms must also contribute significantly. Other possible physical mechanisms for the creation of a sediment supply include volcanism, meteoric impacts, glacial erosion, wind abrasion, mass wasting, and thermal stress (Figure 9; Kocurek and Ewing, 2010). Low temperatures and the near absence of liquid water at the surface of Mars retard chemical weathering, limiting the present-day contribution of such weathered material to the sediment body of dunes.

11.15.2.2.3

Sediment availability

Known factors currently affecting sediment availability on Mars include those acting either from outside or within the

Mars Sediment state

The predominance of wind ripples (Jerolmack et al., 2006), dunes (Hayward et al., 2007), and aeolian cross-strata (Grotzinger et al., 2005; Herkenhoff et al., 2007; Metz et al., 2009) makes it clear that the martian landscape has long been dominated by aeolian depositional processes (Figure 8). Similarities to Earth at the dune- and ripple-scale, which arise owing to the self-organizing nature of the complex system of sediment transport (see below), belie differences in how sediment is generated, supplied, and transported.

11.15.2.2.2

Sediment supply

The large dark dunes on Mars are dominated by mafic minerals (e.g., Rogers and Christensen, 2003; Bandfield, 2002).

Figure 8 Crossbedding exposed in Cape St. Vincent, a promontory jutting into the 750 m diameter Victoria crater, as observed by Opportunity on Mars. Image credit: NASA/JPL/Cornell.

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sand deposits, either shielding or binding the sand, respectively, and thus inhibiting transport. External inhibitors include superimposed ice and crusts. At high latitudes, wintertime CO2 ice accumulations can grow to be as thick as 1.5–2 m (Aharonson et al., 2004), impeding sediment transport until the ice sublimates in the spring. Poorly understood crusts coat the surfaces of ripples at the Mars Exploration Rover (MER) landing sites, increasing the effective friction velocity required for grain saltation (Sullivan et al., 2008); such crusts are likely common on the surface of Mars (e.g., Jakosky and Mellon, 2001). Internal factors such as interstitial ice within highlatitude dunes may slow or halt sand transport, similar to behavior observed in the dunes of the McMurdo Dry Valleys in Antarctica (Bourke et al., 2009). In the distant past on Mars, groundwater and precipitation most likely impacted aeolian transport in Meridiani Planum, as observed by the MER Rover Opportunity, as the effects of groundwater are apparent in the aeolianites exposed by impact and erosion (Grotzinger et al., 2005). Thus ancient sand systems on Mars may have undergone different processes that limited sediment availability.

11.15.2.2.4

Transport capacity

Anemometers on the Viking Landers indicated that prevailing winds blew well below the saltation threshold friction speed, but they may have exceeded it during episodic dust storms (Arvidson et al., 1983). Based on these measurements and a lack of observed bedform migration over the 30 Earth years between Viking and Mars Global Surveyor imaging (e.g., Malin and Edgett, 2001), the general impression has been that saltation and creep are inactive under current climatic conditions. In addition, cratered bedforms suggest that stabilization has occurred in some areas for relatively long periods of time (Reiss et al., 2004; Golombek et al., 2010). However, careful study of overlapping images of dunes began to indicate that changes driven by aeolian processes do occur: some small dome dunes have eroded (Bourke et al., 2008) and slip faces on some barchanoid dunes show temporally varying streaks interpreted as grainflows (Fenton, 2006). In situ observations from the MER show contemporary removal of sand deposits (Geissler et al., 2008) and episodic ripple migration (Sullivan et al., 2008). Images from HiRISE, which reach to a spatial resolution of 25 cm/pixel and resolve meter-sized boulders and ripples, are beginning to show even more evidence of aeolian activity (Chojnacki et al., 2011; Geissler et al., 2010; Silvestro et al., 2010a; Hansen et al., 2011). Each of the above detections occurred at different locations on Mars, indicating that winds are strong enough, at least on occasion, to move sand in many regions. Further investigations may reveal that, like Earth, Mars is a planet with local climates, leading to spatial and temporal variations in capacity of wind transport.

11.15.2.3 11.15.2.3.1

Venus Sediment supply

The two known dune fields are located near an impact crater and tessera (tectonically deformed terrain), which have been the proposed sources of sand for each dune field (Greeley et al., 1992). A potential reason for the lack of observed dunes may be a planetwide paucity of sediment. As on Mars, sand

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grains on Venus are likely composed of mafic minerals weathered from the volcanic bedrock. The surface may have been completely resurfaced by lava flows about 500 Ma ago, and the kinetics of breakdown of basalt by the atmosphere are uncertain (other sand-generating processes such as explosive volcanism, freeze–thaw, glacial action, or fluvial erosion do not occur on present-day Venus). Indeed, the dominant source of sand-sized sediment on Venus may be the ejecta from impact craters; Garvin (1990) calculated that impacts could produce enough fine-grained materials (o1 mm) to form a globally averaged layer as much as 1 m thick.

11.15.2.3.2

Sediment availability

On Venus there is no water present to restrict sand transport. Geologic activity does not appear to be active enough to quickly bury or destroy sand accumulations. Laboratory experiments indicate that the extreme near-surface air temperature could lead to accretion of aeolian grains as they saltate on a surface, with comminuted particles adhering to the surface (Marshall et al., 1991). It is unclear what effect this might have on the long-term survivability of aeolian grains, but it would certainly limit surface abrasion, leading to deposition rather than erosion as sand is transported through a region.

11.15.2.3.3

Transport capacity

In the 1970s, two Soviet spacecrafts landed on Venus and measured wind speeds in the range of 0.5–1 m/ s1 at a height of B1 m (Florensky et al., 1977). Although slow, these winds are strong enough to move sand grains in the high density atmosphere (Greeley et al., 1984b). Because of the high atmospheric density, grains are so readily entrained that midair collisions may ‘‘choke’’ the flow of particles, impeding the process that creates ripples and possibly destroying bedforms (Williams and Greeley, 1994). Thus, surviving bedforms may indicate regions of moderate wind activity; high transport capacity of the wind may indeed limit bedform formation to a degree not seen on other planetary bodies, depositing only poorly sorted sand sheets. Given the above discussion, it is possible that sediment supply and availability, as well as the transport capacity of the wind, all contribute to the low detection of bedforms on Venus. It is also likely that the Magellan radar did not resolve many smaller bedforms. It will take a new mission to Venus to determine more about the nature of aeolian processes on this world.

11.15.2.4 11.15.2.4.1

Titan Sediment supply

Processes considered capable of producing sand grains on Titan include impact cratering and fluvial erosion. Noting that about 40% of the low-latitude half of Titan appears covered in dunes, and using radarclinometric, radiometric, and similarity arguments, Lorenz et al. (2008a) estimated the total dune sand volume to be between 200 and 800 thousand cubic kilometers of material, equivalent to a thickness of several meters over the whole surface. This range of values may be used to constrain formation mechanisms of aeolian sand. For example, an initial estimate of the volume of river channels

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indicates that they cannot produce enough sand to construct the dunes (Lorenz et al., 2008b). Because more heavily eroded areas have since been found (e.g., Malaska et al., 2010), it may be that this calculation should be revisited. The observed abundance of impact craters is likewise unable to provide the required volume: although the crater volume is comparable to the volume of the sand dunes, impact ejecta ranges in size from large blocks to fine dust, and thus cannot have produced all of the observed sand. To understand a more likely source of aeolian sand, other factors and processes unique to Titan must be considered. The optimum particle diameter for saltation in the atmosphere, assuming interparticle cohesion similar to those of terrestrial materials, is about 250 mm (Greeley and Iversen, 1985; Lorenz et al., 1995; Burr et al., 2009). The optically dark appearance of the sand and its spectral characteristics support an organic composition, leading to an alternative proposed source: atmospheric haze particles (Barnes et al., 2008). Conversion of o1 mm haze particles into 250 mm sand grains could occur by sintering over long timescales, or perhaps it may involve cycles of wetting and drying in Titan’s lakes (Aharonson et al., 2009). The latter scenario would require that the sand move from the lakes at high latitudes to the equatorial regions, where the dunes occur. Although modeled wind patterns do indicate net transport toward the equator, there is no evidence for transport from any obvious source region, so that the main provenance of the sand is still unconstrained.

11.15.2.4.2

Sediment availability

Because source regions and transport pathways for Titan’s sand are yet unidentified, it is difficult to determine factors controlling sand availability. Humidity may play a role in controlling sediment availability: Mitchell (2008) shows that methane humidity should vary systematically with latitude, with the tropics becoming desiccated and thus allowing sand to be mobile, whereas higher latitudes may be damper. However, if the hydrocarbons that are thought to comprise the dune sand precipitate directly from the atmosphere, then sediment availability may never play a role in determining the sediment state of these sand seas, because the sand would not be transported along the surface and become subject to processes that limit sediment availability. Once built, the dunes may become stabilized, building linear dunes in a unidirectional wind regime (Rubin and Hesp, 2009), while storing sediment for potential future transport. However, morphological similarity to terrestrial longitudinal dunes (i.e., produced in a bimodal wind regime) suggests that the dunes are formed from noncohesive sand (e.g., Lorenz et al., 2006; Radebaugh et al., 2008), so that postformation stabilization (and reduced sediment availability) is less likely.

11.15.2.4.3

Transport capacity

Doppler tracking of the Huygens probe during its parachute descent indicated winds near the surface of B0.3–1 m s1 (Bird et al., 2005). Although weak, these are comparable with calculations of the saltation threshold, suggesting that aeolian activity may readily occur in the present-day environment. Using a general circulation model, Tokano (2008) suggested that that a low drift potential at higher latitudes may prevent dune formation, another potential explanation for the lack of

dunes poleward of 7301. Further atmospheric modeling by Tokano (2010) suggests that strong winds blowing from directions likely to produce the observed dune morphology occur only shortly after both equinoxes, so that significant sand saltation may occur only for short periods once every 15 (terrestrial) years. This hypothesis is consistent with the observed long, straight dune morphology: large dunes produced by these relatively short-lived wind events would have a long reconstitution time (B300 000 terrestrial years), which would produce long features with few meanders (Tokano, 2010).

11.15.2.5

Depositional Sinks – Continuity at Basin Level

Aeolian sediment drifts and dunes arise wherever there is sufficient wind to transport available sand. Dune fields can exist where the sediment budget (i.e., sediment influx vs. sediment outflux) is negative, neutral, or positive (Wilson, 1973; Mainguet et al., 1984; Kocurek and Havholm, 1994). In the case of a negative sediment budget, dunes may migrate across a surface that is undergoing net erosion (such that the surface elevation drops over time); in the case of a positive sediment budget, dunes may migrate across a surface that is accumulating sediment (such that the surface elevation increases over time). Aerodynamic conditions determine the sediment budget in dry aeolian systems and a near-surface water table is an additional control in wet aeolian systems (Kocurek and Havholm, 1994). Wind deceleration arising from convergent atmospheric circulation patterns, vertical flow expansion in the lee of topographic obstacles, and a spatial increase in surface roughness are some of the aerodynamic conditions that cause a spatial decrease in sediment flux that gives rise to a positive sediment budget. On Earth, many sand seas (with areas 432 000 km2), which account for the majority of the land surface covered by dune sand, form within broad, tectonically stable basins because of these aerodynamic controls. Some of these dune fields maintain a positive sediment budget over a long time period and reflect deposition through multiple Croll–Milankovich cycles (e.g., Nanson et al., 1995; Lancaster et al., 2002). Other (typically smaller) dune fields reflect sediment budgets that are influenced by climate-driven changes in sediment supply, availability, and wind patterns.

11.15.2.5.1

Mars

On Mars most dune fields occur within topographic depressions, consistent with dune field development as dry aeolian systems in a water-limited environment. The relative lack of tectonic activity and erosion has permitted the accumulation of impact craters that host nearly 75% of the dune fields between latitudes 7651 (Hayward et al., 2007). Small dune fields and sand sheets are also common on the floor of the Valles Marineris rift system, a series of canyons that is more than 4000 km long. Dune fields are less common in other topographic sinks, which include tectonic troughs and the largest basins: the northern lowlands, and Hellas and Argyre Planitiae (ancient impact craters 1700 km and 900 km wide, respectively). They rarely occur on open plains away from topographic sinks, although large ripples (which are likely granule ripples) and TARs are more common in such

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locations. The lack of vast sand accumulations has implications for sand formation and transport on Mars: the rate of sand creation may be too low to supply a large volume of sediment, or the grains may not survive long-distance transport.

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11.15.3.1

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Venus and Titan

The two dune fields identified on Venus are located in topographic lows, but otherwise it is not clear why bedforms have accumulated in these locations. It has been speculated that sediment sources are local (Weitz et al., 1994), suggesting that transport distances on Venus are short. The topography of Titan is still poorly known, so that the position of dunes relative to relief cannot be well constrained. However, many linear dunes terminate abruptly at the edges of obstacles or rough terrain, and pick up gradually thereafter. Some obstacles have ‘tails’ of dunes in the downstream direction. This interaction with apparent topography is similar to that observed in the Namib desert, suggesting that sediment on Titan and Earth reacts to topography on similar length scales (Radebaugh et al., 2010).

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Emergent Structures in Depositional Aeolian Landscapes Self-Organization

Dune field patterns form some of the most remarkable landscapes on Earth, Venus, Mars, and Titan, and are widely interpreted as self-organized phenomena arising within the complex system of sediment transport (Hallet, 1990; Werner 1999; Bishop et al., 2002; Kocurek and Ewing, 2005; Baas, 2007). Self-organization of dune fields is the development of a pattern from a nonpatterned state as a result of interactions between the dunes themselves via the wind. Although the fluid/grain properties of the sediment transport system on Mars, Venus, and Titan are different from those on Earth (see Table 1), the emergent patterns are largely similar. Similarities among patterns in such dramatically different environments arise because grain-scale processes are subordinate to dunescale processes, such as dune–dune interactions (e.g., see Figure 10), which determine pattern ordering and operate similarly on both Earth and Mars, and likely Titan (Werner, 1999; Kocurek et al., 2010). Dune field patterns reflect the environmental boundary conditions within which the pattern evolved (Ewing and Kocurek, 2010). Much of the diversity among dune field patterns arises from differences in boundary conditions. Wind regime, which determines crestline orientation (Rubin and Hunter, 1987), and sediment supply, which determines the transition from a barchan dune to a continuous crescentic dune, are examples of familiar boundary conditions on Earth, Mars, and Titan. Ice within dune sediments may play a significant role in determining pattern formation on Mars (Schatz et al., 2006; Feldman et al., 2008; Ewing et al., 2010; Kocurek and Ewing, 2010). On Titan, either the wind regime (Radebaugh et al., 2010) or strong sediment cohesiveness (Rubin and Hesp, 2009) may play a role in the development of the ubiquitous linear dunes.

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11.15.3.2

Dune Field Patterns

Because dune field patterns record the processes of selforganization and environmental boundary conditions, they provide a direct, and generally primary, record of geomorphic process and climatic change at the surface of a planetary body. Dune field patterns may potentially provide one of the most complete records of surface processes operating in the Solar System, including wind direction (Tsoar et al., 1979; Rubin and Hunter, 1987; Hayward et al., 2007), sediment source area (Fenton et al., 2003; Ewing et al., 2010; Silvestro et al., 2010b), dune age (Werner and Kocurek, 1999; Ewing et al., 2006; Derickson et al., 2008), and atmospheric boundary layer depth (Andreotti et al., 2009; Lorenz et al., 2010). Determining wind regime from dune orientation is a primary goal when interpreting dune field patterns because over large spatial scales dune orientation may reflect planetary-scale atmospheric circulation patterns. However, crestlines are oriented as perpendicular as possible to all constructive winds by maximizing sediment transport across the crestline (i.e., gross bedform-normal transport of Rubin and Hunter, 1987; Rubin and Ikeda, 1990). Dune crestlines, therefore, are aligned to the direction of net dune migration, but they do not permit unique identification of the dune-forming winds (except in the case of a unidirectional wind regime). Interpretations of Titan’s wind regime from linear dune field patterns illustrate the difficulty of determining wind direction from crestline orientation. Coarse-resolution radar images of Titan’s surface show very continuous, straightcrested, and regularly spaced lineations (Figure 7), which have been interpreted as linear dunes (Lorenz et al., 2006). Using

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gross bedform-normal transport (Rubin and Hunter, 1987; Rubin and Ikeda, 1990; Werner and Kocurek, 1997) this generic striped pattern could have formed within any wind regime except a bimodal wind in which the divergence angle between the two winds was at around 901, which would form star dunes. The hypotheses for the constructional wind pattern of Titan’s dunes reflect the ambiguity of deciphering the linear pattern, with interpretations of wind regimes ranging from unidirectional to obtuse bimodal (see reviews in Rubin and Hesp, 2009; Tokano, 2010). Although the hypotheses are mostly reasonable depending on the cohesive or noncohesive nature of the dune sediment (Rubin and Hesp, 2009), it is the morphologic signature of the interaction of the dune pattern with topographic obstacles (i.e., an antecedent topography boundary condition) that has provided some constraints on the formative wind directions (Radebaugh et al., 2010; see Figure 7 and Section 11.15.1.3.2). Crest spacing, crest length, and defect density (where defects are defined as breaks in the dune pattern and dune crestline terminations; the defect density is the total number of defect pairs divided by the total crestline length) are dune field-scale variables that can be used to characterize the evolutionary state of a dune field pattern (Werner, 1999; Werner and Kocurek, 1999; Ewing et al., 2006). Typically, pattern evolution occurs with an increase in dune spacing and crest length and a decrease in defect density, such that dune fields with regular, widely spaced, continuous crestlines and few defects represent mature, well-organized patterns. Analysis of the spatial relationships of these pattern variables provides a robust, quantitative basis from which geomorphic and climatic interpretations of a dune field can be made (Ewing et al., 2006, 2010; Beveridge et al., 2006; Derickson et al.,

2008; Bishop, 2007; Wilkins and Ford, 2007; Rachal and Dugas, 2009). With a few exceptions, dune field pattern analysis is relatively unexplored on Mars and Titan (see Figure 11; Bishop, 2007; Ewing et al., 2010; Savage et al., 2010). Bishop (2007) used point-pattern statistics to measure the degree of organization of barchan dunes and barchanoid/crescentic ridges from MOC images in the north polar region of Mars and showed that barchanoid ridges display a greater degree of organization than isolated barchan dunes. Ewing et al. (2010) measured dune crestline spacing, crest length, defect density, and orientation and used geomorphic observations from HiRISE imagery from the central area of Olympia Undae to show that the pattern there represents two generations of pattern construction. Results indicate that the best-organized pattern represents an older generation of dune construction, which is transverse to circumpolar easterly winds. A younger dune generation, which developed with the most recent transporting winds from the northeast and easterly winds, is reworking the older generation. Multigenerational dune field patterns are common on Earth and typically reflect changes in wind regime associated with glacial–interglacial cycles related to the Milankovitch forcing. On Mars and Titan, however, multigenerational patterns appear to be the exception. Surveys of dunes on Mars and Titan indicate that most patterns are simple (cf. Kocurek and Ewing, 2005; Savage et al., 2010), consisting of a single generation of dune construction. This is indicated by a low variability in spacing and crest length populations and a low defect density. Simple patterns typically imply that sediment supply and wind conditions have been constant for an extended period of time. The complex pattern (i.e., multigenerational)

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Figure 11 Examples of pattern analysis in the northern polar dunes on Mars. Reproduced from Ewing, R.C., Peyret, A.-P.B., Kocurek, G., Bourke, M., 2010. Dune field pattern formation and recent transporting winds in the Olympia Undae dune field, north polar region of Mars. Journal of Geophysical Research 115, E08005, doi: 10.1029/2009JE003526, and Bishop, M.A., 2007. Point pattern analysis of north polar crescentic dunes, Mars: a geography of dune self-organization. Icarus 191(1), 151–157, with permission from AGU.

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reported by Ewing et al. (2010) developed with an influx of sediment from the formation of a re-entrant in the sandbearing layers of the north polar ice cap of Mars. The creation of the re-entrant channeled katabatic winds and injected a new supply of sediment into Olympia Undae. In this case, the multigenerational pattern does not appear to be associated with global-scale climate changes, but rather a local change in the topographically forced wind patterns.

11.15.3.3

Bedform Scaling

What determines the size of a bedform? Crest heights and wavelengths (i.e., crest spacing) of aeolian ripples and dunes on Mars, Titan, and Venus range over five orders of magnitude in scale (Breed et al., 1979; Greeley et al., 1992; Lorenz et al., 2006; Hayward et al., 2007). Wavelengths range from centimeters to kilometers, and heights range from centimeters to hundreds of meters. Relating the scaling of these bedform characteristics to a physical length scale within the fundamental fluid/sediment transport system would provide a straightforward way to quantify environments on other worlds via remote imagery. Determining this relationship has been a long standing goal within bedform research and no single satisfactory explanation has yet emerged (Bagnold, 1941; Sharp, 1963; McLean and Smith, 1986; Werner and Kocurek, 1999; Andreotti, 2004); however, much progress has been made recently that relates bedform patterns to aspects of the external length scales and internal, self-organizing dynamics (Claudin and Andreotti, 2006; Andreotti et al., 2009; Pelletier, 2009; Ewing and Kocurek, 2010). Wind ripples constitute the smallest aeolian bedforms and display some of the most regular patterns found on Earth and other worlds. In an early attempt to explain steady-state spacing in ripples, Bagnold suggested that the characteristic wavelength of aeolian ripples was equal to the mean saltation path length. This perspective was challenged by Sharp (1963) with the observation that ripples start small and merge into larger bedforms. In a more recent work, Anderson (1987) suggested that the initial, fastest growing wavelength is equal to about 6 times the mean hop length of the reptating fraction of sand, which is sand splashed away from a high energy impact of a saltating grain. Ultimately, Anderson (1990) concluded that narrow distribution of wavelengths of organized ripples relates to nonlinear interactions, such as merging, of ripples. Anderson’s model, which includes Sharp’s (1963) effect of ‘shadowing’ – the upwind side of a ripple receiving more impacts than the downstream side, embodies the more probabilistic perspective that is characteristic of modern work in aeolian features. A more recent model discussed in Pelletier (2009), based on the compression of streamlines on a wavy bed, predicts that ripples should grow to a steady-state spacing of 3000 times the aerodynamic roughness length. He also argued that in turn, ripples cause a dune-forming instability at a larger scale. Bagnold (1941) originally pointed out the existence of a ‘minimal’ wavelength in dunes, which refers to the wavelength at which dunes form a slip face. Bagnold (1941) suggested this minimal spacing related to the length scale over which sand flux saturates for a given wind. This concept has been built upon in the recent years with attempts to relate various physical

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mechanisms to sand flux saturation (Sauermann et al., 2001; Andreotti et al., 2002; Hersen et al., 2002; Andreotti, 2004; Charru, 2006). In a development of the saturation length concept, Claudin and Andreotti (2006) suggested that the drag length (Andreotti et al., 2002; Hersen et al., 2002), which is the distance required to accelerate a grain in saltation to the fluid velocity, is related to the minimum size of a bedform (i.e., the ‘‘elementary’’ bedform). Furthermore, the relation between the drag length and elementary bedform size holds over several orders of magnitude, including subaqueous dunes, snow dunes, dunes observed on Earth and Mars, and those produced under Venus-like conditions in a wind tunnel. The same analysis applied to Titan predicts meter-scale wavelength bedforms – a spatial scale below the resolution of available radar data of Titan’s surface. Of critical importance to this type of study is determining what constitutes a minimal sized bedform and identification of this type of bedform from imagery of the surface; for example, it is not known whether the dunes observed on Venus and Titan are complex (consisting of smaller bedforms superimposed on larger features) or simple (elementary). With the emergence of a field of migrating dunes, dunes coalesce, grow, and organize into well-developed patterns (see the review in Kocurek et al., 2010). The maximum achievable wavelength of a dune pattern is of considerable debate, but the resolution of this question potentially offers important insights to the timing of dune formation and the surface conditions within which the dunes evolve. Andreotti et al. (2009) proposed that the maximum dune size scales with the thickness of the atmospheric boundary layer, which acts like the free surface of a liquid, acting to ‘cap’ the flow perturbation due to the dune. In an application of this approach, Lorenz et al. (2010) suggested that the boundary layer on Titan correlates with the highly uniform spacing of linear dunes occurring at Titan’s surfaces. In the same spirit of invoking an external template for dune spacing, Pelletier (2009) suggested that steady-state spacing of dunes relates to the wavelength and morphology of superimposed ripples, which create the primary aerodynamic roughness elements on the dune. Werner and Kocurek (1999) employed the nonlinear dynamics of interacting bedforms to suggest that bedforms never reach a true maximum wavelength, but rather asymptotically approach a maximum when the number of defects in a dune field decreases and dunes no longer interact. Ewing and Kocurek (2010) explored this idea using the Werner and Kocurek (1999) model and empirical data relating dune wavelength and dune field area and found that the area of a dune field roughly scales with maximum dune spacing. The area imposes a limit on the number of initial dunes, which limits the number of interactions and maximum dune size. As yet, these concepts have yet to be thoroughly tested and applied to dune fields in the Solar System, but they may prove to be of great value in interpreting the aeolian history of these worlds.

11.15.4

Erosional Landscapes

In addition to mobilizing particles to form depositional landforms, wind erodes the landscape at a variety of scales, leaving

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characteristic features ranging from ventifacts to yardangs. A detailed review of the attributes of and processes that form erosional landscapes is provided in (Chapters 11.14 and 11.8), respectively, in this volume. Because no wind-eroded features have been identified on Titan, discussion here is limited to Venus, Earth, and Mars.

11.15.4.1

Review of Deflation and Abrasion on Earth

Two wind processes, abrasion and deflation, cause erosion. Abrasion is the removal of material from rock or soil by the impact of saltating sand. Deflation is the removal of soil or cohesive sediment due to wind shear and does not require the presence of saltating particles, although the two processes can occur simultaneously. Wind can move coarse sand and gravels by saltationinduced creep and dust by suspension, but it is saltating sand, and on the terrestrial planets, silicate sand in particular, that accounts for most or all abrasion. There has been some discussion in the literature on the role of dust, ice, and snow in abrading rocks, but these can be discounted for several reasons (Laity and Bridges, 2009). The small diameter of dust means that its kinetic energy (KE) upon impact is B104–107 times less than that from sand and therefore produces little or no abrasion. Another factor is that the inertia of sand grains can carry them through the boundary layer to strike a surface, whereas dust will follow the gas streamlines around it (much as rain drops will hit a car’s windshield, but fog droplets may not). Theoretical, field, and wind tunnel studies show that KE flux profiles for saltating sand exhibit distinct maxima several tens of centimeters above the ground (Sharp, 1964, 1980; Anderson, 1986; Wilshire et al., 1981; Liu et al., 2003). In contrast, the KE of dust should increase logarithmically with height, an imprint not occurring on abraded surfaces. Scanning electron microscopy shows that even polished rocks, which have been proposed to form their sheen from dust abrasion, are characterized by chips, gouge marks, and other features consistent with sand impact (Laity and Bridges, 2009). A more detailed discussion of these arguments is contained in Chapter 11.8. The ability for wind shear to move material off a surface can be described by a curve that relates threshold friction speed to particle diameter (Shao and Lu, 2000). The minimum in this curve represents the lowest friction speed at which particles can be mobilized; on Earth this value is on the order of 0.23 m s1 for idealized quartz spherules with a diameter of 100 mm (see Figure 6 in Chapter 11.8). At sizes greater than the minimum, wind shear is balanced against particle weight, such that sand larger than a few hundred microns is mobilized only under extreme wind conditions. At sizes less than the minimum, cohesion and interparticle forces increasingly dominate as particle size decreases, such that dust (i.e., particles a few to tens of microns in diameter) mobilization may require triggering by saltating sand (Peterfreund, 1981; Christensen, 1983) or dust devils (Neubauer 1966; Greeley et al., 2003; Neakrase and Greeley, 2010). On a purely theoretical basis, aeolian erosion can occur wherever there are winds above the saltation threshold and a supply of loose material can be harnessed. These conditions

are clearly met on Earth, where wind velocities of just a few meters per second are able to mobilize sand (for typical surface roughnesses, the wind speed at 1 m height is about 10  the friction speed). However, it is the highest speed winds that transport grains at the fastest speeds and greatest fluxes; ventifacts show alignment with these trends, rather than all winds above threshold. For example, in the heavily abraded Little Cowhole Mountains in the Mojave Desert, ventifacts align with winds with speeds greater than 10 m s1 at a height of 2 m (Bridges et al., 2004), which converts to a friction speed of about 0.6 m s1, or about 3  the minimum threshold (Greeley and Iversen, 1985). Because abrasion requires sand, a supply of this material must be locally available. In all areas for which ventifacts are found, sand either occurs today or was present in the past (see Chapter 11.8). This supply requirement accounts for the location of ventifacts in deserts, periglacial regions, and coastlines – areas where unconsolidated sand is abundant. Similarly, although many yardangs form partially, or in the case of some very soft examples, entirely through deflation, the role of sand is prominent in most cases (Laity, 1994, 2009). As a result, notches above yardang bases are attributable to sand impact (see Chapter 11.14). Local topography influences the direction of the wind and sand transport, causing funneling through troughs and acceleration over swales. This process accounts for the prevalence of ventifacts in such locations (Laity, 1987, 1994). Corridors between yardangs serve as pathways for sand transport, where enhanced abrasion leads to further erosion.

11.15.4.2

Deflation and Abrasion Elsewhere in the Solar System

Mars contains abundant ventifacts and yardangs. Ventifacts (see Figure 5 of Chapter 11.8) have been identified at three rover landing sites to date: ‘Pathfinder’ (Bridges et al., 1999), ‘Spirit’ (Greeley et al., 2006, 2008; Thomson et al., 2008) and ‘Opportunity’ (Sullivan et al., 2005). Initial imagery from Curiosity suggests that ventifacts in Gale crater will soon be reported as well. In general form and local distribution, martian ventifacts are not appreciably different from those on Earth. However, they are generally composed of basalt, reflecting the dominant rock on the planet, although the composition is not uniform; those in Meridiani Planum (‘Opportunity’) are composed of sulfate-rich rocks (Sullivan et al., 2005), and in the Columbia Hills in Gusev Crater (‘Spirit’) ventifacts are made from layered basaltic clastic rocks, which probably have a volcanic origin (Squyres et al., 2007). As on Earth, areas of complex topography, as exemplified by the Columbia Hills, exhibit enhanced abrasion on summits and alignment of ventifact features with winds predicted to have been funneled along troughs formed by local hills (Greeley et al., 2008) and large stabilized ripples (Thomson et al., 2008). Yardangs are abundant on Mars, with a heavy concentration in the Medusae Fossae Formation (MFF; McCauley, 1973; Bradley et al., 2002; Mandt et al., 2008; de Silva et al., 2010; Zimbelman and Griffin, 2010). Figure 12 shows examples of yardangs at the edge of the MFF displaying a

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variety of morphologies. Other areas of the planet also contain yardangs, as well as massive or layered material that appears to have been draped over topography and then exhumed, perhaps multiple times (Malin and Edgett, 2001; Hynek et al., 2002; Edgett, 2005). The MFF is geographically associated with the Tharsis and Elysium volcanic provinces, which may mean that the component material is volcanic, most likely ash or ignimbrite (Ward, 1979; Scott and Tanaka, 1982; Wells and Zimbelman, 1997; Bradley et al., 2002; Hynek et al., 2002). However, other materials, such as lithified dust (‘‘duststone’’; Ruff et al., 2001; Bridges et al., 2010) or dehydrated lags of former dust–ice mixtures (Schultz and Lutz, 1988) have been proposed for the MFF and other wind-eroded mantles. In any case, the presence of yardangs and landscapes undergoing exhumation points to the extensive role of aeolian erosion. The presence of abundant ventifacts and yardangs is consistent with the vast deposits of sand dunes, ripples, and sheets occurring on the planet. Considering the lower gravity and thinner atmosphere of Mars, threshold friction speeds are about an order of magnitude greater than on Earth (see Table 1). When these winds do occur, they are of sufficient magnitude to consistently accelerate sand to velocities that will cause abrasion. However, the frequency of such winds on Mars is on the order of 10–4 (Greeley et al., 1982), in contrast to 0.01–0.2 for winds near or greater than 10 m s1 in typical deserts on Earth (Fryberger, 1979; Bridges et al., 2004; Lancaster, 2004). This indicates that deflation and abrasion should occur much less frequently on

Mars. The fact that they occur, as evidenced by martian geomorphology, is most likely due to the greater exposure age of features on Mars compared to Earth, thereby allowing low frequency events to produce similar work, but integrated over longer timescales. Elsewhere in the Solar System, there is little indication of erosional aeolian landscapes. The evidence for aeolian erosion on Venus is limited to one field of putative yardangs identified near Mead Crater in Magellan radar (see Figure 13; Greeley et al., 1992). Images from the four Venera Landers showed no ventifacts. On Titan, Cassini radar has not revealed yardangs, and the single surface view of Titan provided by Huygens does not show ventifacts. The apparent paucity or lack of aeolian erosion features on these worlds may simply be because the resolution of the radar data is lower than those of orbital Mars images and because surface missions were fewer and lacked mobile platforms. Nevertheless, it may very well be that the lack of abrasion features is real and not simply a reflection of limited data. Surface wind speeds measured on Venus and Titan to date are on the order of 1–2 m s1 (Counselman et al., 1979; Bird et al., 2005). Although above the threshold needed to move sand, the KE of the grains should be low, such that the effectiveness of abrasion will be limited.

11.15.4.3

Comparison Between Earth and Other Bodies

Were there no images of the surfaces of Mars, Venus, or Titan, the predictions made for the role of aeolian erosion would

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timeline (B3 Ga – present according to the chronology of Hartmann and Neukum, 2001).

11.15.5

Unanswered Questions

Despite much study, many rather fundamental questions about aeolian processes on each of these worlds have remained unanswered. A more detailed view of some of these issues is presented in recent literature reviews, should readers desire more information (Bourke et al., 2010; Kocurek and Ewing, 2012). The authors conclude by listing some of the many planetary aeolian mysteries yet to be resolved.

11.15.5.1

1 km Figure 13 A portion of the putative yardang field on Venus. Image courtesy of NASA Planetary Data System Imaging Node, U.S. Geological Survey, Flagstaff, AZ.

probably not be far off the mark. Mars, with abundant sand and occasional high speed winds, displays abundant ventifacts and yardangs. Venus and Titan, with low speed winds, have but limited evidence (yardangs on Venus) for aeolian erosion. Therefore, of all the factors involved in the process, the most critical can be narrowed to just two: 1) supply of sand and 2) winds of sufficient speed and frequency such that, integrated over time, the surface becomes eroded, predominantly through abrasion. In comparing just Earth and Mars, the supply of sand is not an issue, as it is voluminous on both planets. As mentioned above, the main differences between Earth and Mars are the intensity and frequency of winds above which abrasion can occur. The integrated abrasion rate on Mars is probably about 1/100–1/10 that of typical deserts on Earth (a more detailed discussion of this issue is presented in Chapter 11.8). However, given the greater age of surfaces on Mars, such a rate is actually too high. Integrating continuously over a billion years, virtually all rocks and landforms would be eroded away (Greeley et al., 1982). Two factors are likely to reduce the integrated abrasion rate. Unlike on Earth, where fluvial and other processes are active, the production of sand is low in the present martian environment. Therefore, most sand is old and, over time, a large portion of it may have become confined to topographic traps such as craters (Hayward et al., 2007), comminuted during saltation, or otherwise become unavailable for transport (e.g., buried); thus, much of the sand produced on Mars may have been effectively removed from abrading the surface. Additionally, many areas of Mars have undergone multiple episodes of burial and exhumation (Malin and Edgett, 2001; Hynek et al., 2002; Edgett, 2005), such that the exposure age of rocks is much less than the age of rock deposition. Such a hypothesis is consistent with most yardangs being formed in the Amazonian epoch, the most recent epoch as defined by the crater density

Mars

How does the sediment transport system impact the shape, size, and development of bedforms on Mars? Some bedforms occurring on Mars, such as TARs and reticulate bedforms, are not seen elsewhere in the Solar System. Careful study of terrestrial coarse-grained ripples and dunes seems to indicate that some TARs may be distinguishable as either ripples or dunes, but detailed study may be necessary to make this discrimination (Zimbelman et al., 2010). In the case of reticulate bedforms, it is likely that they are formed from dust aggregates, particles not commonly found in abundance beyond Mars, making analog study challenging. Both are examples of depositional aeolian bedforms that are distinct in morphology, and probably in grain size and composition, from the large mafic dunes that are more similar to terrestrial bedforms. Their role in the sedimentary history of Mars is still unclear. What is the timing of dune formation on Mars? Was dune formation slow and continuous or did it occur rapidly from short-lived events? When and where does sediment transport occur on Mars? What types of aeolian features are active on Mars today? Increasingly there are studies showing that some bedforms on Mars are active, but others are eroded and likely inactive. Each aeolian system reflects the climatic and sedimentary history that has shaped it, so that understanding the distribution and degree of aeolian activity is crucial to understanding how aeolian processes have shaped and continue to shape the landscape. What are the major sand sources on Mars? When did the sand production occur on Mars? It is not clear if a single process dominates sand production on Mars, or if different processes have dominated at certain points in the planet’s history.

11.15.5.2

Venus

What aeolian features exist on Venus? The tentatively identified dune fields and the single yardang field may in fact be nonaeolian structures. Further data are necessary to confirm these interpretations, as well as the presence of ‘microdunes.’ How does the environment influence sediment transport and bedform morphology? The laboratory experiments showing that aeolian processes may be different on Venus are yet to be verified. Does adhesion cause sediment accumulation during saltation? Do strong winds destroy bedforms? What implications does this have for the aeolian environment on

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Venus, and how can bedform morphology be used to learn more about the sedimentary history, a rare topic of study on this planet? Why are dunes on Venus so rare? A potential reason for the lack of observed dunes may be a planetwide paucity of sediment. The surface may have been completely resurfaced by lava flows about 500 Ma ago, and the kinetics of breakdown of basalt by the Venusian atmosphere are uncertain. Other sandgenerating processes such as freeze–thaw, glacial action, or fluvial erosion do not occur on the present-day Venus. Indeed, the dominant source of sand-sized sediment may be ejecta from impact craters. Garvin (1990) calculated that impacts could produce materials fine-grained enough (o1 mm) to form at most a globally averaged layer 1 m thick. Noting that the two Venusian dune fields likely account for only about 1000 km3 of sediment in total, this estimated sediment budget may be sufficient to produce the observed dune fields. Future Venus exploration (Bullock et al., 2009) is likely to include platforms able to return image data on spatial scales 1–2 orders of magnitude better than those of Magellan, employing either higher-resolution (possibly interferometric) radar from orbit, or near-infrared imaging from a balloon near the surface. Such future missions will likely reveal many more duneforms than are presently known, and would likely reinvigorate aeolian studies on Venus.

Nevertheless, the available data and subsequent modeling provide some insight into how aeolian processes differ from one planetary body to another. The locations of sand seas on Mars and Titan indicate that wind circulation patterns and the distribution of climates are dramatically different from those on Earth. Variations in both local and global factors, such as ground ice and water, topography, grain cohesion and survivability in transport, the rate of sediment transport, climate shifts, and sediment sources, can strongly influence both the rate of bedform development and bedform morphology in ways not observed on Earth. Although these boundary conditions vary quite dramatically from one world to another, they do form recognizable bedforms and dune fields that can then be studied to extract the sedimentary history of the regions where they form. Erosional landforms on Mars and Venus are similar to bedforms in this respect: the processes and timescales that produce them may differ from those that create these features on Earth, but the final results are familiar ventifacts, yardangs, and eroded surfaces. As with all planetary science, the other aeolian worlds in the Solar System may be regarded as laboratories for aeolian conditions not reproducible on Earth, extending the range of known environmental responses to extreme circumstances and providing new perspectives on terrestrial aeolian landforms.

11.15.5.3

References

Titan

What is the source and nature of the sand? What implications do these have for sediment transport and dune morphology? The origin of dune sand on Titan is not well understood, but the most likely source is sintering of atmospheric haze particles. Such a potentially exotic source may have equally unusual implications for the formation and behavior of dunes, because the sand supply may be dependent on conditions of local production as well as transport. What causes the dunes to be restricted to low latitudes? Titan’s dune fields are controlled in latitude more than any other aeolian feature in the Solar System, and yet it is not clear what causes this rather striking pattern. Determining whether sediment supply, availability, or wind transport capacity is the dominant controlling factor will have global implications for atmospheric circulation and chemistry.

11.15.6

Conclusions

Despite the growing repository of planetary data, we still know very little about many fundamental aspects of aeolian processes beyond Earth. Limited by the paucity of surface meteorological data, particle transport cannot be correlated with measured winds, although transport events have been observed on Mars. On Venus and Titan, data limitations have precluded identification of bedforms smaller than B100 m; identification of elemental bedforms on these worlds would provide a test of the recent theories of bedform pattern formation. These observational limitations clearly constrain what is known about sediment transport and bedform morphology on these worlds.

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Zimbelman, J.R., Griffin, L.J., 2010. HiRISE images of yardangs and sinuous ridges in the lower member of the Medusae Fossae Formation, Mars. Icarus 205, 198–210. Zimbelman, J.R., Williams, S.H., Johnston, A.K., 2010. Cross-sectional profiles of ripples, megaripples, and dunes: a method for discriminating between formational mechanisms. Proceedings of the Second International Planetary Dunes Workshop, Alamosa, CO, Abstract #2013.

Biographical Sketch Lori K Fenton received her B.S. in physics and astronomy at the University of Maryland, College Park, and her Ph.D. in planetary science from the California Institute of Technology. Following a postdoctoral position at Arizona State University, she joined the Carl Sagan Center at the SETI Institute as a research scientist in 2006. Her research has focused on aeolian geomorphology and boundary layer meteorology on Mars, involving analysis of spacecraft data and utilization of several types of atmospheric models. In particular, her primary interest lies in determining how planetary aeolian features are influenced by and record both current weather patterns and changes in climate. She may be reached at [email protected].

Ryan Ewing studied geology at the University of Texas at Austin and obtained his doctorate in 2009 under the supervision of Professor Gary Kocurek. He then worked as a postdoctoral research scholar at Princeton University and the California Institute of Technology. He began his current position as Professor in Geology at the University of Alabama in 2011. His research emphasizes using wind-blown sedimentary systems to unravel the geomorphic and sedimentary histories of modern and ancient Earth, Mars, and Titan.

Nathan Bridges’ research focuses on surface processes in the Solar System, principally wind erosion and transport. His work incorporates field studies, wind tunnel investigations, theoretical treatments, and planetary data analysis. He is a coinvestigator on the ‘Mars Reconnaissance Orbiter’ and ‘Mars Science Laboratory’ missions. Dr. Bridges worked at the Jet Propulsion Laboratory from 1997 to 2009 and is currently a Senior Scientist at the Applied Physics Laboratory.

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Ralph Lorenz has a B.Eng. in Aerospace Systems Engineering from the University of Southampton in the UK and got a Ph.D. in Physics in 1994 from the University of Kent at Canterbury. He worked in 1990–91 for the European Space Agency on the design of the Huygens probe and during his Ph.D. research designed and built its penetrometer instrument that 12 years later measured the mechanical properties of Titan’s surface when Huygens landed in January 2005. From 1994 to 2006 he worked as a planetary scientist at the Lunar and Planetary Laboratory, University of Arizona, with particular interests in Titan, Mars, planetary climate, nonequilibrium thermodynamics, aerospace vehicles, and radar. He continues to work on those topics at the Johns Hopkins University Applied Physics Laboratory in Laurel, MD. He is author or coauthor of several books, including ’Lifting Titan’s Veil,’ ’Spinning Flight,’ and ’Space Systems Failures,’ as well as over 180 publications in refereed journals.

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